Currently, 3rd generation (3G) cellular communication systems are being developed to further enhance the communication services provided to mobile phone UEs (user equipment). The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) technology, namely Wideband Code Division Multiple Access (WCDMA). Carrier frequencies are used for uplink transmissions, i.e. transmissions from a mobile wireless communication unit (often referred to as wireless subscriber communication unit or UEs (in 3rd generation systems) to the communication infrastructure via a wireless serving base station (referred to as a Node B in 3rd generation systems) and downlink transmissions, i.e. transmissions from the communication infrastructure to the mobile wireless communication unit via a wireless serving base station (e.g. Node B). A further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of Universal Mobile Telecommunication System (UMTS), can be found in WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
It is known that a cellular hierarchy consists of a macro cell for wide area geographic coverage and micro, pico and femto cells used for small localised coverage within the macro geographic structure. In some cases the micro, pico and femto cells may use the same frequency as that of the macro cell. Since the micro, pico and femto cells are used for in-fill coverage, their radiative antenna units will not be sharing the same physical site as the antennae used in the macro sector.
An example of a known cellular network plan 175 as often used in a macro cell deployment is illustrated in FIG. 1. The cellular network plan utilises cells 180 in a “honeycomb” structure, where a base station (Node B) would employ conventional (passive) antennas comprising multiple antenna elements to support communications within each cell. Cells are divided into sectors 185. Typically, three sectors exist per cell, corresponding to approximately 120° coverage per sector. One sector corresponds to a radiation pattern of a single conventional antenna whose horizontal azimuth beam pattern of +/−65° HPBW (Half Power beam width) maximally covers the sector. Outside of a main lobe of an antenna beam employed within the sector, the signals are spatially filtered and significantly attenuated. Conventional network planning and passive antenna array solutions process all incoming signals with a common fixed beam pattern. This receive processing, based on signals received within the geographic area identified by the antenna main lobe, tends to dictate a corresponding common beam pattern for transmitter operation. Thus, an identical radio frequency (RF) footprint is used for both receive (Rx) and transmit (Tx) operation.
Six sector cells, though less common, are utilised in some network configurations. CDMA and WCDMA technologies are able to use a single modulated RF carrier frequency for all uplink users on all cells and sectors within those cells. Likewise, a single carrier frequency is used for downlink on all cells in the network and all sectors within these cells. Furthermore, each sector antenna radio frequency (RF) signal is processed independently by its respective receiver or transmitter.
In conventional antenna systems for cellular communications, the transmit/receive beam pattern is often controllable using electromechanical beam steering elements, such as mechanical phase shifters. Beam steering is currently performed remotely by manipulation of electromechanical elements of an antenna array located at the top of the antenna mast. Electromechanical beam manipulation is limited to minor changes in tilt angle (typically up to 10°). Beam Steering in the horizontal azimuth plane, is implemented either manually or using electromechanical adjustments. Electromechanical phase shift beam manipulation (horizontal or vertical) does not extensively change the main beam shape. When performing beam steering, signals from conventional panel antennas elements are combined together prior to receive signal processing. However, with such vertical/horizontal azimuth plane beam steering it is known that angle of arrival information of incoming signals is not capable of being discerned.
Efficient use of energy and infrastructure resources is known to be a primary focus in the operational and capital expenditure in cellular networks. Furthermore, guaranteeing link performance improves QoE (Quality of Experience) to the user enabling ARPU (Average Revenue Per User) yielding services to be maximised. In order to enable Network Operators to optimise infrastructure configuration, information pertaining to the location of users within a sector, cell and cluster of cells would be useful. To date, Network Operators have conducted field trials concurrent with performing iterative updates to a most commonly used network configuration, as a means to network optimisation. This is known to be a slow and expensive process. Furthermore, such slow changes can not be dynamically adapted to user population changes within the network. Using conventional antenna systems, user geographic density can only be estimated for users as processed by a cell or cluster of cells. Sometimes triangulation methods can be attempted to determine where users exist within a cell or cluster of cells, based on handover information.
Receive (Rx) beam-forming using antenna arrays depends on the ability to constructively add incident signals on each of the antenna elements in a way that coherently adds those from the desired direction. Thus, incident signals that are not from the desired direction will be incoherently added, and thus will not experience the same processing gain. The term ‘coherency’ implies that the signals will have substantially the same phase angle when added in the beam forming process. In contrast, thermal noise from multiple sources exhibits incoherent properties. Thus, when thermal noise is added to the incident signals, the signals from multiple sources do not experience the same processing gain as a coherent desired signal.
Conversely, in transmit active antenna arrays, the signals are coherently combined in the ‘air’ within the intended beam pattern as electromagnetic (EM) signals. In this manner, they are arranged to arrive coherently at the intended mobile station (MS) (e.g. UE) receiver.
In order to address the limited flexibility associated with electromechanical phase shifter beam manipulation on an antenna array located at the top of the antenna mast, there has been recent interest in the use of smart or active antenna technology. This is a radio technology where the antenna system has dedicated signal processing per antenna array element or co-located antenna and signal processing units. The active antenna technologies fall into three broad families.
(i) multi-antenna systems (MAS);
(ii) radiohead with or without multiple in-multiple out (MIMO) signals; and
(iii) radio-array.
It is known that active array antenna technology is able to facilitate independent and variable beam patterns in both uplink and downlink directions. Modern air interface protocols, such as WCDMA, allow multiple UEs to simultaneously transmit to the base-station on the uplink on a single carrier frequency. A minimal limit of signal to noise per bit (Eb/No) ratio is required on the uplink channel to ensure an adequate received signal bit error rate (BER). This implies that the higher the data rate for a particular wireless subscriber communication unit, otherwise referred to as user equipment (UE) at least a proportionally better Carrier to Interference plus Noise Ratio (CINR) is required to maintain Eb/No.
As the cell becomes more loaded, all UEs have to compensate by increasing their transmit power, so as to maintain Eb/No at the Node B receiver, since other UEs will add to the interference level. Likewise UEs requiring higher data rate services will require all UEs to compensate for higher Eb/No required of the Node B. This increase in transmit power by the UE can be detrimental to network performance. The increased power levels in the cell propagate to neighbouring cells forcing that cell to increase power levels to maintain Eb/No even though it may not be loaded heavily.
This effect poses a number of problems to WCDMA networks. For example, the noise floor of the communication cell is increased, which in turn causes a propagation effect on the network and reduces data transmission rates. Furthermore, UE devices consequently consume more power, as uplink transmissions are required to operate at higher power levels. In addition, limitations exist on maximum throughput per cell as a result of dynamic range limitations imposed on UE transmissions.
In a Node B antenna array, the received radio frequency (RF) signal from a single UE cannot be discerned without demodulation of the composite (constructively added) signal. Individual receive beam-forming for a specific user is not feasible, since there is likely to be multiple received signals of the same power from different UEs simultaneously at the antenna array. Even if only a few UEs are utilising the Node B at any point in time, the likelihood is that the signals received at the Node B would be below the noise floor of the Node B's receiver. As known, the processing gain of a WCDMA receiver implies that such signals can be extracted from the noise floor for further processing. This, however, still requires at least a partial demodulation process.
U.S. Pat. No. 5,889,494 describes a passive beamforming system for cellular communications, such as AMPS, in order to address the problem of re-use factor of channel frequencies within a cellular network. U.S. Pat. No. 5,889,494 utilises a 360° sector, and proposes a scanning apparatus that selects the antenna beams for processing. In U.S. Pat. No. 5,889,494 the scanning apparatus describes a selection device of fixed beams being processed and does not disclose any technique for dynamically changing a beam.
U.S. Pat. No. 7,072,692 describes a communication system that exploits spatial processing by incorporating a transmit beam pattern based on the incident uplink power angle of arrival in an associated time slot of the uplink channel. The method proposed in U.S. Pat. No. 7,072,692 estimates the uplink channel power by means of measuring the power per beam, with eight beams required per sector. This corresponds to a beam width generation of +/−7.5°. However, U.S. Pat. No. 7,072,692 fails to describe any realizable implementation of how to generate such narrow beams. Beamform generation of this accuracy would require a large array of antenna elements, which is not practical from a cost/power or size perspective with current wireless cellular systems.